U.S. Pat. No. 4,808,887 publicizes a power supply unit for starting fluorescent lamps, which consists of HF filters, a rectification network, push-pull oscillation circuits, series-connected resonance circuits and two groups of parallel-connected diodes. By using HF filters, the unit is intended to eleminate the interference on the power transmission network caused by higher order harmonics produced by capacitors with large capacitance in the voltage regulating circuit in the rectifying network. However, the final results of this type of circuits are not satisfactory.
European Patent EP 0075176 publicizes another power supply unit for discharging lamps, which features almost identical HF filters, rectifying network and resonance circuit, and excitation of the fluorescent lamp by means of a bridge-circuit output, its aim being to avoid phase shift and increase the power factor.
Since in both the above-mentioned circuits capacitors with high capacitance values are used in the voltage regulating circuit of the rectifying network, it is necessary to add a number of HF filters at the input end of the rectifying network. In spite of this, high-order harmonics caused by large the capacitance cannot be completely eliminated. Therefore, the capacitance value of the voltage regulating circuit has to be reduced without affecting the operation of the circuit. The reducing of the capacitance value will result in a significant reduction of the components of high-order harmonics produced by the circuit. The object of the present invention is to reduce the capacitive impedance at the input end of the circuit, increase the matching of the inductive impedance and the capacitive impedance at the input end, thus minimizing the components of high-order harmonics produced by the circuit.
The object of the present invention is to reduce the capacitive impedance at the input end of the circuit, increase the matching of the inductive impedance and the capacitive impedance at the input end, thus minimizing the components of high-order harmonics produced in the circuit, improving the current waveform at the input end to the extent of being near a sinusoidal wave, increasing the power factor and weakening the high-order harmonics of source current to the degree that they no longer impact the power network.
Another object of the present invention is to enable normal operation of a circuit with very lower filtering capacitor and prevent abnormal operation due to lowered capacitance at the input end and poor voltage regulation characteristics by means of an LC network connected ahead rectifying network and filtering, isolating and energy-storage capacitors collocated.
A further object of the present invention is to prohibit the operation when the voltage and current become abnormal at the input end of the power network by means of a protective network connected between the rectifying network and the oscillator output circuit.